The practical utility of a semi-conductor material in many electronic device applications, such as solar cells, photo detectors, xerographic photoreceptors or vidicon targets, depends on the ability of the semiconductor material to absorb light. All semiconductor materials are weakly absorbing or transparent at long wavelengths and strongly absorbing at short wavelengths. The transition from absorbing to transparent occurs at a wavelength for which the photon energy h.nu. is roughly equal to the band gap of the semiconductor. In direct gap crystalline semiconductors, such as GaAs for example, the transition from transparent to absorbing is abrupt, occuring over a small range of photon energy (about 0.05 ev). On the other hand, in indirect gap crystalline semiconductors, of which crystalline Si is an example, or in amorphous semiconductors such as amorphous selenium or amorphous hydrogenated silicon, the optical absorption threshold is relatively broad and the transition from transparent to absorbing occurs over a relatively broad range of wavelengths or equivalently of photon energies. In the amorphous semiconductor case, the width of the transition region is typically of the order of 0.2 ev.
In this intermediate wavelength region these semiconductor materials are not good absorbers of light. One solution for semiconductor devices in which complete absorption is needed, such as solar cells, is simply to make the semiconductor material thicker. However, this approach can have serious drawbacks. First more semiconductor material is required, so that the material cost is higher. Secondly, the collection of all the electron hole pairs generated in a solar cell made from a thick layer of semiconductor requires that the carrier recombination lifetime be higher, and hence the electronic quality of the semiconductor be better than when the semiconductor material is thin.
Light trapping has been proposed in the past as a solution to this problem for self-supporting wafer-type solar cells and photodetectors. Namely by causing weakly absorbed light to make many passes through the semiconductor by special surface structuring, the absorption of light near the absorption threshold can be increased with no increase in material thickness. In one approach, Redfield, U.S. Pat. No. 3,973,994, the back surface of the semiconductor is faceted with a sawtooth pattern and coated with a reflector so that light incident through the front surface is reflected from the back surface at an oblique angle with a long pathlength inside the material. In another approach, St. John, U.S. Pat. No. 3,487,223, the back surface of a silicon wafer is roughened by sandblasting. Here light that is transmitted through the smooth front surface of the wafer is scattered into a range of solid angles when it hits the rough back surface of the wafer. The scattered light has a low probability of escape because of the small escape cone for light in a high index material immersed in a low index media such as air.
In the present invention, light is trapped in semiconductors, deposited by the techniques of thin film deposition. In this invention the semiconductor material is deposited on a roughened substrate whose surface texture, can be substantially larger than the thickness of the semiconductor film. As a result of the substrate texture light is trapped inside the film by scattering, probably at both surfaces of the film and by subsequent total internal reflection. In a thin film material with a relatively high index of refraction, such as hydrogenated amorphous silicon, the light trapping effect can increase the absorption of weakly absorbed light by more than an order of magnitude.